Mass spectrometer of high detection efficiency

ABSTRACT

The ion source of a mass spectrometer is designed to draw ions from a relatively large volume through a diaphragm with a relatively large opening into a magnetic field in a wedge shaped space between diverging plane pole faces, entering such space parallel to the line of intersection of the pole face planes near the wide end of said space. The ions are focused after 90* deflection into an exit slit or diaphragm on the line of intersection of the pole faces. Certain parameters for the position of the exit slit are given.

United States Patent [1 1 Liebl et al.

[451 Oct. 15, 1974 MASS SPECTROMETER OF HIGH DETECTION EFFICIENCYInventors: Helmuth Liebl, Eching; Klaus sa s rr, M n h; 1101 1.

Germany Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V.,Gottingen, Germany Filed: Dec. 26, 1973 Appl. N0.: 427,490

Related U.S. Application Data Continuation of Ser. No. 292,332, Sept.26, 1972, abandoned.

Assignee:

Foreign Application Priority Data Oct. 5, 1971 Germany 2149716 U.S. Cl250/298, 250/281, 250/427 Int. Cl. H0lj 39/34 Field of Search 250/282,281, 290, 298,

[56] References Cited UNITED STATES PATENTS 3,286,187 11/1966 Gaber250/427 3,421,035 1/1969 Brubaker... 250/41.95 B

3,445,650 5/1969 Liebl 250/49.5 B 3,705,998 12/1972 Jennings et a1.250/427 3,761,707 9/1973 Liebl 250/298 Primary ExaminerJames W. LawrenceAssistant Examiner-B. C. Anderson Attorney, Agent, or FirmFlynn &Frishauf 5 7 ABSTRACT The ion source of a mass spectrometer is designedto draw ions from a relatively large volumethrough a diaphragm with arelatively large opening into a magnetic field in a wedge shaped spacebetween diverging plane pole faces, entering such space parallel to theline of intersection of the'pole face planes near the wide end of saidspace. The ions are focused after 90 deflection into an exit slit ordiaphragm on the line of intersection of the pole faces. Certainparameters for the position of the exit slit are given.

6 Claims, 2 Drawing Figures MASS SPECTROMETER OF HIGH DETECTIONEFFICIENCY This is a continuation, of application Ser. No.

292,332, filed Sept. 26, 1912 now abandonegl The present inventionrelates to a mass spectrometer of high detection efficiency and moreparticularly to a I mass spectrometer of the type having an ion sourcein which a gas to be analyzed is ionized by electron bombardment, anapparatus for producing a substantially static magnetic field throughwhich the ions produced in the ion source and projected therefrom by anelectric field through a first diaphragm are deflected on the basis oftheir effective mass so that ions of different effective mass arefocused along a focus line at different places and there enter an iondetection devie in which the ions to be detected enter through adiaphragm suitably positioned in the focus line.

In known types of mass spectrometers of high detection efficiency,particularly those used for analysis of residual gas or for detection ofleaks in vacuum systems, the effective magnetic field provides theanalysis by mass by projecting the intake slit ion-optically on the exitslit. The mass resolution thus depends primarily on the ratio of thedeflection radius to the sum of the slit widths. Since in the case ofresidual gas analysis apparatus and'the like a compact form ofconstruction is desired, the deflection radius and likewise the intakeslit width cannot be made so large as would be desired with referenceonly to the mass resolution. And since, moreover, the ion jet may have aspreading angle of only a few degrees, the known mass spectrometers ofthis type can detect ions only from a small ionization volume in the ionsource. For a given ionization density the obtainable detectionefficiency is, however, proportional to the volume from which the ionscan be detected. The detection efficiency of the known residual gasanalyzers therefore leaves much to be desired.

The object of the present invention is accordingly to provide a massspectrometer of high detection efficiency in which ions can be drawnfrom a large ionization volume in spite of a compact form ofconstruction.

SUBJECT MATTER OF THE PRESENT INVENTION:

Briefly, the ion source in which the gas to be analyzed is ionized byelectron bombardment includes an open metallic helix, such as a wirehelix, a source of electrons external to the helix biasedelectro-negatively with respect to the helix and two open gridsperpendicular to the axis of the helix respectively adjacent the twoends of the helix, one at the positive pole of the power supply and theother one slightly more negative. An acceleration voltage is providedbetween the less positive of these two grids and an adjacent diaphragmwhich also has an open grid across its opening and which serves as theintake diaphragm of the mass spectrometer proper. This diaphragm has anopening that is relatively large compared so that of the outputdiaphragm. The equipment for applying the magnetic field includes twopole pieces having mutually inclined pole faces lying in planes thatintersect in a line parallel to the axis of thehelix of the ion source.The pole pieces also have two mutually perpendicular sides lying in thesame mutually perpendicular planes. One of these planes intersects thethroat of the diverging pole faces and is parallel to the line ofintersection of the pole face planes and at a distance'A therefrom. Theother of these two mutually perpendicular planes is perpendicular to theaxis of the helix of the ion source and hence also of the incoming ionjet. The distance between that axis and the line of intersection of thepole face planes is substantially 2.10 A Finally, the exit diaphragm isbisected by the aforesaid line of intersection of the pole face planesand is offset from the plane extending the side of the pole faces whichis adjacent to the intake diaphragm and perpendicular to its axis by adistance equal to 1.35 A,,.

By these arrangements the result is obtained that ions can be drawn froma large ionization volume in the ion source and the intake diaphragm orslit through which the ions proceed from the ion source into themagnetic field can be made very large in comparison to residual gasanalyzers heretofore known, so that according to the invention a massspectrometer can be provided which has a detection efficiency of aboutone order of magnitude better than the conventional residual gasanalyzers. Y

The acceleration voltage, the magnetic field, or both may be madevariable. The electron source is located outside the helix and ispreferably a hairpin type thermionic cathode, biased substantiallynegative relative to the helix and the grids at its ends, butconsiderably less negative than the diaphragm adjacent one end of thehelix which serves as an accelerating electrode. An electrostatic screenmay enclose the electron source as well as the helix, including the backend of the helix, and this screen is preferably at about the potentialof the electron source or slightly more negative. An illustrativeembodiment of the invention is described below in further detail withreference to the accompanying drawing in which:

FIG. 1 is a greatly simplified plan view of a mass spectrometeraccording to the invention, and I FIG. 2 is a side view of a portion ofthe mass spectrometer shown in FIG. 1.

The embodiment shown in FIGS. 1 and 2 for illustration of the inventionincludes a mass analyzer 11 which works with a magnetic field that isproduced between two plane pole faces that are inclined with respecttoeach other so as to form'a wedge shaped magnetic gap. The lines of forceof the field, as shown in FIG. 2 by arrows, are circular arcs having ascenter the line of intersection 10 of the two planes containing the polefaces 11a and 11b respectively. The field strength between the polepieces is inversely proportional to the distance from the intersectionline 10. Charged particles describe cycloidal paths in such a field. Itis known (H. Liebl, J. Appl. Phys., vol. 38 (1967) pages 5,277, 5283)that when a parallel jet of charged particles enters perpendicularly tothe magnetic field at an offset distance A 2.10 A from the intersectionline 10, with the momentum p, it will be focused after a deflection at apoint on the intersection line if the field strength is so adjusted thatat the distance A 2.10 A, from the intersection line it has the valueB,, 1,35 p/Ae In the above expression: p particle momentum e particlecharge A distance of the intersection line 10 from the pole piece wallsurface 110, which is parallel to the intersection line 10.

The focusing effect follows a second approximation: the distance of thefocus from the entrance edge of the pole pieces is substantially 1.35A,,.

At the location of the focus an output diaphragm 12 in provided, forexample in the form of a slit, behind which an ion detector, for examplean ion capture device 13, is located for detecting the ions penetratingthrough the gap of the diaphragm l2 and for producing a correspondingelectrical output signal at an output terminal 14 for processing in theusual way.

The ion source designated 20 as a whole in the drawing comprises acylindrical wire helix 22 and an electron source 21 outside the helixand extending parallel to the helix axis (in the simplest case in theform of a hairpin filament connected to a suitable source of heatingsurrent 15). At the two ends of the-helix are parallel open wire gridsor similar reticular structures 23 and 24, perpendicular to the axis 22aof the helix 22 and preferably connected with the adjacent ends of thehelix. An electrostatic screen 26, which may be of wire screening in cupshape, surrounds the elements just named, the electron source 21 as wellas the helix 22 and the grids 23 and 24.

Spaced somewhat away from the grid 24 is a first diaphragm 25, theopening of which is bridged by another open wire grid 25a parallel tothe grid 24. The diaphragm 25 serves as an accelerating electrode forthe ion source and as the intake diaphragm or slit for the magneticfield portion of the mass spectrometer.

v The opening of the intake diaphragm 25 is relatively large, its areabeing of the order of magnitude of the crosssectional area of the helix22. Preferably the area of the opening is at least 50 percent on thearea of the cross-section of the helix 22.

The electrodes of the ion source are connected during operation of themass spectrometer with a voltage source 16, which is shown onlydiagrammatically in the drawing- It includes for example a voltagedivider adapted to supply the voltages specified in FIG. 1.

The mass spectrometer above described operates as follows:

The electrons emitted by electron source 21 are accelerated by thepositive bias voltage of helix 22 relative to electron source 21 andoscillate a few times through the helix until they finally land on it.In consequence the gas molecules inside the helix are ionized byelectron bombardment. The oscillation of the-electrons just mentionedincreases the effective path of the electrons, as is known in theparticular context of the Bayard-Alpert ionizationmanometers. Incontrast to the operation of the ionization manometers just mentioned,however, in the ion source of the present invention a small voltage(preferably less than percent and usually even smaller than 2 percent ofthe acceleration voltage U between grid 24 and diaphragm 25) is appliedbetween the parallel open grids 23 and 24 at the ends of the spiral 22,so that a weak homogeneous electrostatic field is present on the insideof the helix. This field is so directed that the ions produced byelectron bombardment are propelled towards grid 24. Between the latterand the diaphragm 25 the actual acceleration voltage U (preferably avariable voltage in the neighborhood of, for example, 500 volts) isapplied to produce an electrostatic acceleration field between the grids24 and 25a, which are arranged in parallel planes. The ion projectedthrough the diaphragm 25 produce a parallel stream or jet that entersthe magnetic field between the pole faces 11a and 11b at their edge 11d.

Typical potentials for the electrodes of the ion source are, withrespect to ground potential 0 volts:

Diaphragm 25 0 V Grid 24 +500 V Grid 23 +5 I0 V Electron source 21 +300V Screen 26 +290 V The theoretical mass resolution is AM/M AU/U S/A 1.40

In the above expression:

AU energy spread of the ions U acceleration voltage S width of the slitshaped opening of the exit diaphragm 12 in the plane of deflection Adistance of the exit diaphragm 12 from the field edge 0 angular spreadof the parallel particle stream or jet at entrance into magnetic field.

The energy spread AU arises primarily from the small voltage appliedbetween the ends of the helix and causes the ions to arise at somewhatdifferent potential. The angle spread 0 comes into play in thecalculation of the mass resolution in the place of the entrance slitwidth which comes up in the corresponding formula for conventionalequipment. The angular spread 0 is composed primarily of alignmenterrors of the grids 24 and 25d and their deviation from parallel planerelation and also the penetration of fields through these grids.

In a practical example of construction of equipment according to theinvention, a partial pressure detection efficiency of 10 A/Torr wasobtained for a half value resolution of 30. This detection SensitivityiEaboiit one order of magnitude better than that of most conventionalresidual gas analyzers.

The embodiment from which the above results was obtained had thefollowing physical parameters:

B,, variable, e.g.

730 Gauss for mass 2 (H 2,720 Gauss for mass 28 (N,+);

3,410 Gauss for mass 44 (CO angle between the pole faces 11a and 11b 8diameter of helix 22 1.5 cm

distance between grids 23 and 24 1.5 cm

diameter of opening in diaphragm 25 1 cm The adjustment of the massspectrometer for a desired effective ion mass (ion mass/ion charge) canbe accomplished by variation of the magnetic field strength and/orvariation of the acceleration voltage applied between grids 24 and 25a.The acceleration voltage can readily be varied by an adjustable tap of apotentiometer in the voltage source 16. For variation of the magneticfield B A a magnet coil 28 may be used which is connected to a source 29of variable current and is associated with a magnetic yoke llj which isonly schematically represented in FIG. 2 as forming part of the magneticcircuit which includes the pole pieces 11 and the pole faces 11a and11b.

Although the invention has been described with reference to a singlespecific embodiment, it is to be understood that modifications andvariations may be made within the inventive concept without departingfrom the spirit of the invention. In particular, although the element 22is described as a helix, it is clear that other cylindrical open cagesof wire, rods, etc., would function in an equivalent fashion, and theuse herein of the work helix" must be taken with that understandmg.

We claim:

1. A mass spectrometer of high detection efficiency comprising:

an ion source (20) including a chamber in which a rarified gas to beanalyzed is adapted to be ionized, a metallic open helix (22)surrounding a portion of said chamber, a source of electrons (21)outside said helix, open grids (23, 24) adjacent to and obstructing thetwo ends of said helix (22), a first diaphragm (25) spaced from one endof said helix (22) centered axially thereto, an open grid (25a) acrossthe opening of said first diaphragm, each of the aforesaid grids (23,24, 25a) perpendicular to the axis (22a) of said helix (22), and meansfor applying electric potential (16) so as to make said grid (23)adjacent the end of said helix (22) fatthest from said first diaphragm(25) most positive,

said grid (24) adjacent the other end of said helix (22) slightly lesspositive, said helix (22) at a potential intermediate that of said endgrids (23, 24), said electron source (21) considerably less positive,and said first diaphragm (25) and its grid (25a), most negative, thevoltage between said end grids (23, 24) of said helix being less than 5percent of the acceleration voltage between the said open grid (24) atthe end of said helix (22) nearer said first diaphragm (25 and the opengrid (25a) across said first diaphragm;

means for producing a substantially steady magnetic field (11, 28, 29)directed at right angles to the axis (22a) of said helix (22) of saidion source (20) including two pole pieces (11) having diverging polefaces (11a, 11b) lying in planes intersecting in a line parallel to theaxis (22a) of said helix (22) of said ion source (20) and each havingalso first and second mutually perpendicular sides (11c, 11d) formingedges with said pole faces (11a, 11b), said first sides (11d) beingperpendicular to the axis (22a) of said helix (22) and said second sides(110), being parallel to the line of intersection (10) of said pole faceplanes and respectively intersecting said diverging pole faces (11a,11b) in lines defining their edge of nearest approach and hence alsodefining the throat of the space enclosed by said diverging faces, and asecond diaphragm (12) in the form of a slit bisected by said line ofintersection (10) of said pole face planes, said second diaphragm (12)being at a distance A from a boundary plane comprising said second sidesof said pole pieces which is parallel to said line of intersection (10)as aforesaid, being also offset by a distance of substantially 1.35 Afrom a plane comprising said first sides (11d) of said pole pieces, andbeing further offset by a distance of substantially 2.10 A from saidaxis (22a) of said helix (22). 2. A mass spectrometer as defined inclaim 1 in which said second diaphragm (12) has an opening which issmall compared'to the opening of said first diaphragm (25) and in whichthe voltage between the open grids (23, 24) at the ends of said helix isnot more than 2 percent of said acceleration voltage.

3. A mass spectrometer as defined in claim 1 in which said helix (22),said source of electrons (21) and said open grids (23, 24) at the endsof said helix are surrounded by an electrostatic screen (26) which alsoencloses the end of said helix (22) farthest from said first diaphragm(25) and in which said electrostatic screen (26) is connected to saidmeans for applying electric potential (16) so that it is at a potentialin the neighborhood of that of said source of electrons (21).

4. A mass spectrometer as defined in claim 3 in which said source ofelectrons (21) is a thermionic cathode of substantially hairpin shapeand in which said electrostatic screen (26) is connected to said meansfor applying of potential (16) so as to maintain it at a voltageslightly more negative than the potential at which said source ofelectrons (21) is maintained.

5. A mass spectrometer as defined in claim 1 in which said open grids(23, 24) adjacent to the extr'emeties of said helix (22) areelectrically connected to the respectively adjacent ends of said helix(22).

6. A mass spectrometer as defined in claim 1 in which there are providedmeans for varying either the acceleration voltage or the magnetic fieldstrength, or both, said acceleration voltage being defined as a voltagebetween said first diaphragm (25) and its grid (25a) and that one (24)of said open grids (23, 24) which is adjacent the end of said helix (22)nearer said first diaphragm (25). r

1. A mass spectrometer of high detection efficiency comprising: an ionsource (20) including a chamber in which a rarified gas to be analyzedis adapted to be ionized, a metallic open helix (22) surrounding aportion of said chamber, a source of electrons (21) outside said helix,open grids (23, 24) adjacent to and obstructing the two ends of saidhelix (22), a first diaphragm (25) spaced from one end of said helix(22) centered axially thereto, an open grid (25a) across the opening ofsaid first diaphragm, each of the aforesaid grids (23, 24, 25a)perpendicular to the axis (22a) of said helix (22), and means forapplying electric potential (16) so as to make said grid (23) adjacentthe end of said helix (22) farthest from sid first diaphragm (25) mOstpositive, said grid (24) adjacent the other end of said helix (22)slightly less positive, said helix (22) at a potential intermediate thatof said end grids (23, 24), said electron source (21) considerably lesspositive, and said frst diaphragm (25) and its grid (25i a), mostnegative, the voltage between said end grids (23, 24) of said helixbeing less than 5 percent of the acceleration voltage between the saidopen grid (24) at the end of said helix (22) nearer said first diaphragm(25) and the open grid (25a) across said first diaphragm; means forproducing a substantially steady magnetic field (11, 28, 29) directed atright angles to the axis (22a) of said helix (22) of said ion source(20) including two pole pieces (11) having diverging pole faces (11a,11b) lying in planes intersecting in a line (10) parallel to the axis(22a) of said helix (22) of said ion source (20) and each having alsofirst and second mutually perpendicular sides (11c, 11d) forming edgeswith said pole faces (11a, 11b), said first sides (11d) beingperpendicular to the axis (22a) of said helix (22) and said second sides(11c), being parallel to the line of intersection (10) of said pole faceplanes and respectively intersecting said diverging pole faces (11a,11b) in lines defining their edge of nearest approach and hence alsodefining the throat of the space enclosed by said diverging faces, and asecond diaphragm (12) in the form of a slit bisected by said line ofintersection (10) of said pole face planes, said second diaphragm (12)being at a distance A0 from a boundary plane comprising said secondsides (11c) of said pole pieces which is parallel to said line ofintersection (10) as aforesaid, being also offset by a distance ofsubstantially 1.35 A0 from a plane comprising said first sides (11d) ofsaid pole pieces, and being further offset by a distance ofsubstantially 2.10 A0 from said axis (22a) of said helix (22).
 2. A massspectrometer as defined in claim 1 in which said second diaphragm (12)has an opening which is small compared to the opening of said firstdiaphragm (25) and in which the voltage between the open grids (23, 24)at the ends of said helix is not more than 2 percent of saidacceleration voltage.
 3. A mass spectrometer as defined in claim 1 inwhich said helix (22), said source of electrons (21) and said open grids(23, 24) at the ends of said helix are surrounded by an electrostaticscreen (26) which also encloses the end of said helix (22) farthest fromsaid first diaphragm (25) and in which said electrostatic screen (26) isconnected to said means for applying electric potential (16) so that itis at a potential in the neighborhood of that of said source ofelectrons (21).
 4. A mass spectrometer as defined in claim 3 in whichsaid source of electrons (21) is a thermionic cathode of substantiallyhairpin shape and in which said electrostatic screen (26) is connectedto said means for applying of potential (16) so as to maintain it at avoltage slightly more negative than the potential at which said sourceof electrons (21) is maintained.
 5. A mass spectrometer as defined inclaim 1 in which said open grids (23, 24) adjacent to the extremeties ofsaid helix (22) are electrically connected to the respectively adjacentends of said helix (22).
 6. A mass spectrometer as defined in claim 1 inwhich there are provided means for varying either the accelerationvoltage or the magnetic field strength, or both, said accelerationvoltage being defined as a voltage between said first diaphragm (25) andits grid (25a) and that one (24) of said open grids (23, 24) which isadjacent the end of said helix (22) nearer said first diaphragm (25).